Heretofore various types of sound barriers have been disclosed for attenuating noise emanating from a traffic area such as a highway. The simplest approach to prevent the transmission of traffic area noise to adjacent areas is the utilization of a simple wall or plate. Barriers consisting of a "plate" having elastic properties and a known thickness, affect a sound field by diffraction of sound waves around the barrier and by refraction and transmission of the sound waves. The latter two effects can be achieved with only limited success using conventional earthwork or solid, upright barriers. The diffracted sound field in the areas of the shadow zone of the barrier relative to the sound field in the absence of the barrier determines the overall effective attenuation. Prediction of the theoretical attenuation may be obtained by the well-known Fresnel integral equations.
It has been suggested that barriers be attributed with a maximum attenuation of 15 dB, due to the influence of diffraction effects over the barrier. With simple barriers, maximum noise level reductions appear to be achievable only at extreme wall heights (greater than 12 feet) and at the higher frequencies (greater than 1000 Hz).
The principal disadvantages of conventional barriers for traffic noise attenuation may be summarized as follows: (1) effective sound reduction is dependent upon barrier height, (2) barrier heights of 25 feet or more (such as would be required to achieve attenuations of 20 dB or more) do not blend aesthetically with the surrounding landscape, (3) construction costs for high level barriers (such as earth berms, depressed roadways, and concrete walls) are extremely costly, and (4) the motorist has the impression that he is captured within a "tunnel" and therefore loses his perspective on distance and speed.
The sound barrier of the present invention overcomes the aforementioned disadvantages of conventional barriers and obtains its employment in noise attenuation by means of a novel and improved method of utilizing the Fresnel effect. Attenuation is provided as a programmed function of height above the ground, with the greatest attention being provided near the ground (where it is most needed) and the least at the top of the array. This graduated attenuation is provided by acoustically treated pickets or splitters which are widest at ground level and taper upwardly to a pointed terminus. In a "picket" embodiment, the pickets are erected with their flat, absorptive surfaces normal to the direction of sound propagation. In a "splitter" embodiment, the splitters are erected with their flat, absorptive surfaces parallel to the direction of sound propagation, and function as acoustically lined ducts.
It is therefore an object of the present invention to provide a novel and improved sound barrier.
It is a further object of the present invention to provide a novel and improved sound barrier which allows the transmission of light and is aesthetically acceptable as compared with solid, wall-like barriers.
Still another object of the invention is to provide a novel and improved sound barrier particularly suited for use either vertically or horizontally disposed along a highway or freeway, or other heavily traveled thoroughfare, or along the side of other noise producing sources, and which is generally superior to similar devices of the prior art.